I can't give you a similar limit for the nvidia chip but it's reasonably safe to conclude it's max is going to parallel that of the CPU. Consequently 75C isn't too high but anything you can do to lower those temps is going to beneficial long term.
up to 80C is pretty good, 90C is a bit hot, and most cards will begin heat throttling at about 100C. GPU temps are allowed to be significantly higher than CPU temps.